1. Field of the Invention
The present invention relates to a mobile terminal, such as a car telephone or a portable telephone, and a mobile communication system.
2. Description of the Related Art
Since portable telephones are designed to operate on relatively low-capacity secondary batteries or primary batteries for reduced size and weight, their maximum transmitter output power is limited to about 1 W. On the other hand, car telephones, which can take power from an onboard secondary battery having a relatively large capacity, have a transmitter output power of about 3 to 5 W. It is known to provide a configuration that enables such a car telephone to be used as a booster for a portable telephone.
In a mobile communication system, each base station measures the field strength of the power radiated from a mobile terminal such as a portable telephone or a car telephone, and controls the transmitter output of the mobile terminal based on the result of the measurement. In a usual configuration, the base station recognizes the station class mark (SCM) of the mobile terminal to learn its maximum allowable level, and when the transmitter output power of the mobile terminal has been increased to the maximum allowable level under the control of the base station, no further increase in the transmitter power is instructed but zone switching is performed. For this purpose, each mobile terminal transmits its station class mark to the base station at the beginning of a call. However, the station class mark transmitted when a call is initiated directly from the portable telephone is different from that transmitted when a call is initiated with a booster connected as described above. Also, there may occur a situation where the portable telephone is connected to or disconnected from the booster during the call.
For example, consider the situation where a voice communication is being performed with a portable telephone connected to a car telephone booster, when the car arrive at the destination and the user gets out of the car. If the user wants to continue the ongoing communication, he will disconnect the portable telephone from the booster. In that case, however, since the base station cannot recognize that the portable telephone has been disconnected from the booster during the call, the base station continues to control the transmitter power on the basis of the station class mark of the booster transmitted at the beginning of the call. If the reception field strength at the base station drops and control is performed instructing the mobile terminal to increase the transmitter power to a level greater than the transmitter power of the portable telephone, the portable telephone fails to conform to the control, and the base station determines that a fault condition has occurred in the portable telephone and disconnects the call.
Therefore, in the prior known system, if the user has to disconnect his portable telephone from a booster during a call, he terminates the call and then performs a call initiating operation once again, whereby the station class mark of the portable telephone is transmitted to the base station which then performs control of the transmitter output of the portable telephone on the basis of the new station class mark. The prior known system thus has the problem that a call has to be terminated before communication can be resumed normally.
Conversely, consider the situation where a voice communication is initiated from the portable telephone and the user gets into a car where he connects the portable telephone to the booster to continue the communication. In this case also, since the station class mark held at the base station is different from the actual station class mark, there arises a problem in that the transmitter power may be raised to a level higher than the base station expects.